This section describes background subject matter related to the disclosed embodiments of the present invention. There is no intention, either expressed or implied, that the background art discussed in this section legally constitutes prior art.
Nanostructuring is necessary for many present applications and industries and for new technologies and future advanced products, such as wire grid polarizers for flat panel displays or architectural glass. Nanostructured substrates may be fabricated using techniques such as e-beam direct writing, deep UV lithography, nanosphere lithography, nanoimprint lithography, near-field phase shift lithography, and plasmonic lithography, for example.
A method of nanopatterning large areas of rigid and flexible substrate materials based on near-field optical lithography is described in International Patent Application Publication WO2009094009 and U.S. Patent Application Publication US20090297989, both of which are incorporated herein by reference. In this technique, a rotatable mask is used to controllably expose a radiation-sensitive material. Typically the rotatable mask comprises a cylinder or cone. The nanopatterning technique makes use of near-field photolithography, where the mask used to pattern the substrate is in contact with the substrate. Near-field photolithography may make use of an elastomeric phase-shifting mask, or may employ surface plasmon technology, where a rotating cylinder surface comprises metal nano holes or nanoparticles. In one implementation such a mask is a near-field phase-shift mask. Near-field phase shift lithography involves exposure of a photoresist layer to ultraviolet (UV) light that passes through an elastomeric phase mask while the mask is in conformal contact with a photoresist. Bringing an elastomeric phase mask into contact with a thin layer of photoresist causes the photoresist to “wet” the surface of the contact surface of the mask. Passing UV light through the mask while it is in contact with the photoresist exposes the photoresist to the distribution of light intensity that develops at the surface of the mask. A phase mask is formed with a depth of relief that is designed to modulate the phase of the transmitted light by π radians. As a result of the phase modulation, a local null in the intensity appears at step edges in the relief pattern formed on the mask. When a positive photoresist is used, exposure through such a mask, followed by development, yields a line of photoresist with a width equal to the characteristic width of the null in intensity. For 365 nm (near UV) light in combination with a conventional photoresist, the width of the null in intensity is approximately 100 nm. A PDMS mask can be used to form a conformal, atomic scale contact with a layer of photoresist. This contact is established spontaneously upon contact, without applied pressure. Generalized adhesion forces guide this process and provide a simple and convenient method of aligning the mask in angle and position in the direction normal to the photoresist surface, to establish perfect contact. There is no physical gap with respect to the photoresist. PDMS is transparent to UV light with wavelengths greater than 300 nm. Passing light from a mercury lamp (where the main spectral lines are at 355-365 nm) through the PDMS while it is in conformal contact with a layer of photoresist exposes the photoresist to the intensity distribution that forms at the mask.
Another implementation of the mask may include surface plasmon technology in which a patterned metal layer or film is laminated or deposited onto the outer surface of a mask. The metal layer or film has a specific series of through nanoholes. In another embodiment of surface plasmon technology, a layer of metal nanoparticles is deposited on the transparent mask's outer surface, to achieve the surface plasmons by enhanced nanopatterning.
Unfortunately, it is difficult to laminate or deposit metal onto a mask made of elastomeric materials such as polydimethyl siloxane (PDMS).
It is within this context that embodiments of the present invention arise.